I attempt to show in this thesis that Beethoven's musical concepts were deeply enriched through the influence of Freemasonry and other types of ancient philosophical and esoteric thought. The composer integrated these concepts into his world view as well as his music. Evidence of this philosophical/musical synthesis can be seen by comparing his personal writings with certain of his compositions that reflect such thought and attitudes. Three works are examined from this point of view: the Bagatelle, op. 119, No. 7, the Piano Sonata in C minor, op. 111, and the String Quartet in C-sharp minor, op. 131.